1. Field of the Invention
This invention relates to a magnetic recording head and a magnetic recording apparatus suitable for realizing data storage with high recording density, high recording capacity, and high data transfer rate.
2. Background Art
In the 1990s, the practical application of MR (magnetoresistive effect) heads and GMR (giant magnetoresistive effect) heads triggered a dramatic increase in the recording density and recording capacity of HDD (hard disk drive). However, in the early 2000s, the problem of thermal fluctuations in magnetic recording media became manifest, and hence the increase of recording density temporarily slowed down. Nevertheless, perpendicular magnetic recording, which is in principle more advantageous to high-density recording than longitudinal magnetic recording, was put into practical use in 2005. It serves as an engine for the increase of HDD recording density, which exhibits an annual growth rate of approximately 40% these days.
Furthermore, the latest demonstration experiments have achieved a recording density exceeding 400 Gbits/inch2. If the development continues steadily, the recording density is expected to realize 1 Tbits/inch2 around 2012. However, it is considered that such a high recording density is not easy to realize even by using perpendicular magnetic recording because the problem of thermal fluctuations becomes manifest again.
As a recording technique possibly solving the above problem, “radio-frequency magnetic field assisted recording” is proposed. In radio-frequency magnetic field assisted recording, a radio-frequency magnetic field near the resonance frequency of the magnetic recording medium, which is sufficiently higher than the recording signal frequency, is locally applied. This produces resonance in the magnetic recording medium, which decreases the coercivity (Hc) of the magnetic recording medium subjected to the radio-frequency magnetic field to less than half the original coercivity. Thus, superposition of a radio-frequency magnetic field on the recording magnetic field enables magnetic recording onto a magnetic recording medium having higher coercivity (Hc) and higher magnetic anisotropy energy (Ku) (e.g., U.S. Pat. No. 6,011,664). However, the technique disclosed in U.S. Pat. No. 6,011,664 uses a coil to generate a radio-frequency magnetic field, and it is difficult to efficiently apply a radio-frequency magnetic field in high-density recording.
Techniques based on a spin torque oscillator are also proposed as a means for generating a radio-frequency magnetic field (e.g., US Patent Application Publication No. 2005/0023938 and US Patent Application Publication No. 2005/0219771). In the techniques disclosed in US Patent Application Publication No. 2005/0023938 and US Patent Application Publication No. 2005/0219771, the spin torque oscillator comprises a spin injection layer, an intermediate layer, a magnetic layer (oscillation layer), and electrodes. When a DC current is passed through the spin torque oscillator via the electrodes, the spin torque generated by the spin injection layer produces ferromagnetic resonance in the magnetization of the magnetic layer (oscillation layer). Consequently, a radio-frequency magnetic field is generated from the spin torque oscillator.
Because the spin torque oscillator has a size of approximately several ten nanometers, the generated radio-frequency magnetic field is localized within approximately several ten nanometers around the spin torque oscillator. Furthermore, the perpendicularly magnetized magnetic recording medium can be efficiently resonated by the longitudinal (in-plane) component of the radio-frequency magnetic field, allowing a significant decrease in the coercivity of the magnetic recording medium. Consequently, high-density magnetic recording is performed only in a portion where the recording magnetic field of the main magnetic pole overlaps the radio-frequency magnetic field of the spin torque oscillator, allowing use of magnetic recording media having high coercivity (Hc) and high magnetic anisotropy energy (Ku). Thus the problem of thermal fluctuations in high-density recording can be avoided.
On the other hand, there is also a technique for using an oblique recording magnetic field to perform recording on a magnetic recording medium having high coercivity (Hc). According to the Stoner-Wohlfarth model, a magnetic recording medium having high coercivity (Hc) can be reversed in magnetization under a 45°-oriented magnetic field. In perpendicular magnetic recording, an oblique recording magnetic field can be generated from a surface of the recording magnetic pole intersecting with the recording medium facing surface. Furthermore, to generate an oblique magnetic field having steep variation in magnetic field strength, it is effective to add an auxiliary magnetic pole near the recording magnetic pole. By adjusting the gap between the surface of the recording magnetic pole intersecting with the recording medium facing surface and the surface of the auxiliary magnetic pole intersecting with the recording medium facing surface, the magnetic field produced in the recording medium can be inclined to steepen the strength variation. Hence a magnetic recording head having a recording magnetic pole and an auxiliary magnetic pole enables high-density recording, allowing use of a magnetic recording medium having higher coercivity (Hc) and higher magnetic anisotropy energy (Ku).